Method of casting magnesium



Patented May 29, 1934 UNITED STATES PATENT OFFICE METHOD OF CASTINGMAGNESIUM No Drawing. Application February 20, 1933, Serial No. 657,675

Claims.

casting magnesium and alloys thereof. One object of my invention is topermit casting such metals in water-bound or green sand molds and thelike, whereby the metal is protected from attack by air or by themoisture in the mold. Other objects and advantages will appear as thedescription proceeds. For convenience the term magnesium is used hereinand in the appended claims to mean not only magnesium but also alloysthereof in which the magnesium predominates.

The production of commercially acceptable castings of magnesium presentsa problem which is unique in foundry practice due to the aflinity forboth oxygen and nitrogen which this metal possesses, especially atelevated temperatures. In the molten state, at temperatures employed incasting, the metal is capable of combining spontaneously with oxygen,which may be derived either'from the air or from substances containingfree or combined oxygen that are incontact with the metal, such as wateror steam or other compounds in the molding material. The burning oroxidation of molten magnesium in an atmosphere of steam or in contactwith water may be at times so violent as to approach explosiveness. Thusit is practically impossible to make a sound casting of magnesium in asimple water-bound mold of conventional type, such as is used forcasting other metals.

Various methods have been proposed by which it is sought to overcome thetendency for water M or other oxygen compounds in a mold to attackmagnesium when cast therein. Heretofore, for

example, it has been suggested'to incorporate sulphur or boric acid inwater-bound sand molds for casting magnesium to overcome the harmfuleffects of air or water vapor on the metal. However, these substanceshave proved to be not wholly satisfactory,- inasmuch as when asufficient amount of eithersulphur or boric-acid is added to the moldingsand to prevent oxidation F of a casting of relatively heavy section,the thin sections are liable to be pitted. On the other hand, if theamount of agent is reduced to a point where pitting of thin sections isreduced, heavier sections may be insufficiently protected fromoxidation. Other types of protective agents, namely, ammonium fluorideand ammonium silicofluoride, have been proposed in United States PatentNo. 1,825,242 to prevent the burning of magnesium when cast inwaterbound molds, and at the same time to produce 5 acorrosion-resistant film upon the surface of the The present inventionrelates to a method of of the sand with the resulting distortion of themolds. Accordingly, the aforementioned patent proposes to reduceswelling and loss of ammonia by adding hydrofluoric acid or oxalic acidto the molding sand in addition to ammonium fluoride or siliccfiuoride,These materials, however, are

undesirable inasmuch as hydrogen fluoride is especially corrosive, whileoxalic acid is toxic.

Briefly stated, the problem encountered in satisfactorily castingmagnesium in temporary water-bound molds involves overcomingthedetrimental effect of one or more of the following factors:-(1)oxidation, which may be due to attack by air, water vapor, or other,oxygen compounds, or even by the sand itself; {(2) the formationof pitsand blowholes due to gas evolution when the molten metal is poured intothe mold; (3) the formation of superficial coatings or films of anon-metallic nature on the surface of the castings which give them anunattractive appearance.

I have discovered that certain new and improved agents, when mixed withwater-tempered foundry sand or the like, are more stable and permanenttherewith, especially under the effects of the heat of casting in moldsmade fromsuch mixtures, than agents heretofore employed and. do notpossess'their undesirable properties. My new agents do not causeswelling of foundry sand and, due to their stability, the sand mixturescontaining them may be used repeatedly without replenishing the agenttherein. Magnesium castings made by proper procedure in temporary moldstreated with my new agents are free from pits, blowholes or similardefects, are substantially free from oxidized areas and are ofcommercially acceptable appearance. The principle of my inventionconsists in including in the mold material, at least for the partsadjacent to the metal cast therein, a sufficient quantity of one of mynew agents to provide protection from oxidation or burning.

To the accomplishment of the foregoing and related ends, the invention,then, consists of the method hereinafter more fully described andparticularly pointed out in the claims, the following descriptionsetting forth in detail several forms of molds and modes of carrying outthe invention, such disclosed molds and modes illustrating, however, butseveral of the various ways in which the principle of .the invention maybe used.

My invention is based upon the discovery that the fluophosphoric acidsand their volatile salts, such as the ammonium salts thereof, when mixedwith moist foundry sand, not only overcome the tendency for oxidation orburning of magnesium when cast therein but also permit making castingsof either relatively heavy or light section substantially withoutpitting, blowholes or similar flaws, and of commercially acceptableappearance. The agents which I have found to be suitable for the purposearez-hexafluophosphoric acid, HPFs; ammonium hexafiuophosphate, NHdPFG;difluophosphoric acid, HPOzFz; ammonium difluophosphate, NI-I4PO2F2;monofluophosphoric acid, HzPOsF; ammonium monofluophosphate, (NHOzPOsF.These new agents are apparently less volatile under casting conditionsthan those hereinbefore mentioned, so that their effectiveness is notlost after repeated use of the sand, while they do not have adetrimental effect on the molding quality of foundry sands.

The treated sand may be used repeatedly to produce castings ofgood'appearance and free from the usual defects without replenishing theagent.

The amount of such agents or mixtures thereof which may be employedvaries with the size and type of castings to'be made as well as with thenature of the molding material. In making large castings agreaterproportion of the agent may be required than when relativelysmall castings are made. Normally the foundry molding material may beintimately mixed with from 0.5 to 10 per cent by weightof any one of thecompounds aforementioned, although usually about 3 to 6 per cent issatisfactory. Mixtures of two or more of these substances may beemployed, the sum of which preferably does not exceed about 10 per cent.For castings having relatively thin sections from 2 to 5 per cent of theagent may be suflicient, while for the averagerun of castingsapproximately 4 per cent of same may be used. It is understood, however,that the quantity of one or more of such agents employed in a given caseis a matter of judgment, governed more or less by the size and characterof the casting.

Naturallythe effectiveness of any agent for use as hereinbeforedescribed varies with the permeability of the molding material, it beingsomewhat greater with sand of high permeability than with those ofrelatively low permeability. The molding material should, therefore, besufllciently open or permeable to permit the free escape out of the moldof gases which are formed by contact with the hot metal as it is poured.In general, I have found that a permeability factor of 30 or more givessatisfactory results, although in manycases good results maybe obtainedwith sands of somewhat lower permeability. The definition of the termpermeability as used herein and the method of measuring its numericalvalue are described in the bulletin Testing and Grading Foundry Sandpublished by The American Foundrymans Association, March 1931.

By way of illustrating a preferred mode 03 carrying out my invention, Imade an intimate mixture of water-tempered foundry sand having apermeability factor of about 25 and about 3 per cent of ammoniumhexafiuophosphate, and formed the mixture into a mold. Then I pouredinto the mold a magnesium alloy consisting of approximately 4 per centof aluminum, 0.3 per cent of manganese, the balance being magnesium, ata temperature between 1300 and 1400 F. The surface of the resultingcasting was smooth, free from pits, blowholes, and oxidized areas. Nofurther superficial treatment, such as sandblasting, was required torender the casting suitable for use.

In similar manner I made a mold from watertempered foundry sandcontaining about 4 per cent of a mixture of 50 per cent of ammoniummonofiuophosphate and 50 per cent of ammo,- nium difiuophosphate, andpoured therein a magnesium alloy. The resulting casting wassubstantially free from oxidized areas and pitting and of commerciallyacceptable appearance. When I employed a relatively small proportion ofhexafluophosphoric acid in the molding material in similar manner inmakinga magnesium alloy casting, the surface thereof was free from oxideand of commercially acceptable appearance.

While I have described-my invention more particularly in terms of awater-tempered molding sand having in admixture therewith one or more ofthe agents above specified, my invention is not necessarily limitedthereto inasmuch as these agents are effective to prevent oxidation orother defects in'casting magnesium when used in mold facings or insolution sprayed upon the inner surface of the mold. For example moldfacings may be made from foundry sand with which is mixed one or more ofmy new agents, and the mixture applied to the pattern, in the usualmanner. A suitable method of coating the mold cavity or core piecesis-to form a solution or suspension of one or more of my new agents,preferably in a non-aqueous liquid such as carbon tetrachloride, andthen spray or otherwise apply the mixture to the mold parts which comeinto con-v tact with the metal. The use of my new agents, furthermore,is not necessarily limited to molds made from sand since other moldablematerials, such as=-graphite, pulverized carbon, coke, cinders, or thelike, may be substituted for the sand.

Other modes of applying the principle of my invention may be employedinstead of those explained, change being made as regards the stepsherein disclosed, provided those stated by any of the following claimsor their equivalent be employed.

I therefore particularly point out and distinctly claim asmyfinventiom- 1. The method of casting magnesium and alloys thereof intemporary molds'which comprises incorporating in water-tempered moldingmaterial an agent selected from the group consisting ofhexafluophosphoric acid, monofiuophosloys thereof which comprisesincorporating ammonium monofluophosphate in water-tempered foundry sand,forming a mold of the mixture and casting the metal in the mold.

4. The method of casting magnesium and a1- 5 loys thereof whichcomprises incorporating ammonium difiuophosphate in water-temperedfoundry sand, forming a mold of the mixture and casting the metal in themold.

HANS A. REIMERS.

